Method of forming a metal silicide transparent conductive electrode

ABSTRACT

A method of forming a metal silicide nanowire network that includes multiple metal silicide nanowires fused together in a disorderly arrangement on a substrate. The metal silicide nanowire network can be formed by applying a solution that contains silicon nanowires onto the substrate, forming a metal layer on the silicon nanowires, and performing a silicidation anneal such that the metal silicide nanowires are fused together in a disorderly arrangement, forming a mesh. After the silicidation anneal is performed, any unreacted silicon or metal can be selectively removed.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application claims the benefit under 35 U.S.C. § 120 as acontinuation of U.S. patent application Ser. No. 14/828,663, filed onAug. 18, 2015, which is a divisional of U.S. patent application Ser. No.13/313,101, filed on Dec. 7, 2011, now U.S. Pat. No. 9,312,426, issuedon Apr. 12, 2016, the entire teachings of which are incorporated hereinby reference.

BACKGROUND

The embodiments disclosed herein relate to transparent conductiveelectrodes and, more particularly, to a structure with a metal silicidetransparent conductive electrode and a method of forming the structure.

Transparent conductive electrodes (also referred to herein astransparent conducting films (TCFs)) are films that are both opticallytransparent allowing transmittance of, for example, 80% or more ofincident light and electrically conductive. Those skilled in the artwill recognize that transparent conductive electrodes are oftenincorporated into modern devices, such as flat panel displays, touchpanels, solar cells, light emitting diodes (LEDs), organicoptoelectronic devices, etc. Currently, the material of choice for theseelectrodes is Indium Tin Oxide (ITO). Unfortunately, Indium isrelatively rare, making its use cost prohibitive in light of the globaldemand for transparent conductive electrodes. Additionally, transparentconductive electrodes formed from Indium may be overly brittle andtoxic. Therefore, there is a need in the art for a transparentconductive electrode structure and method of forming the structure thatis more commercially viable, robust, and safe to use.

SUMMARY

Disclosed herein are embodiments of a structure with a metal silicidetransparent conductive electrode, which is commercially viable, robust,and safe to use (i.e., non-toxic) and, thus, optimal for incorporationinto modern devices, such as flat panel displays, touch panels, solarcells, light emitting diodes (LEDs), organic optoelectronic devices,etc. Specifically, the structure can comprise a substrate (e.g., a glassor plastic substrate) and a transparent conducting film (i.e., atransparent conducting electrode) on that substrate. The transparentconducting film can be made up of a metal silicide nanowire network. Forexample, in one embodiment, the metal silicide nanowire network cancomprise multiple metal silicide nanowires fused together in adisorderly arrangement on the substrate. In another embodiment, themetal silicide nanowire network can comprise multiple metal silicidenanowires fused together in a grid on the substrate.

Also disclosed herein are method embodiments for forming such astructure. Specifically, the methods can comprise providing a substrateand forming a metal silicide nanowire network for a transparentconducting film on that substrate.

In one embodiment the metal silicide nanowire network can be formed suchthat it comprises multiple metal silicide nanowires fused together in adisorderly arrangement on the substrate. Specifically, this metalsilicide nanowire network can be formed by: applying a solution thatcontains silicon nanowires onto the substrate; forming a metal layer onthe silicon nanowires; and performing a silicidation anneal.Alternatively, the metal silicide network can be formed by: forming ametal layer on the substrate; applying a solution that contains siliconnanowires onto the metal layer; and performing a silicidation anneal.Alternatively, the metal silicide network can be formed by: applying asolution that contains metal nanowires onto the substrate; forming asilicon layer on the metal nanowires; and performing a silicidationanneal. Alternatively, the metal silicide network can be formed by:forming a silicon layer on the substrate; applying a solution thatcontains metal nanowires onto the silicon layer; and performing asilicidation anneal. In any case, after the silicidation anneal isperformed, any unreacted silicon or metal can be selectively removed.

In another embodiment the metal silicide nanowire network can be formedsuch that it comprises multiple metal silicide nanowires fused togetherin a grid on the substrate. Specifically, this metal silicide nanowirenetwork can be formed by: forming a metal layer on the substrate;printing a first set of multiple parallel silicon nanowires on the metallayer; printing a second set of multiple parallel silicon nanowires onthe metal layer over the first set of multiple parallel siliconnanowires such that the first set is perpendicular to the second set;and performing a silicidation anneal. Alternatively, this metal silicidenanowire network can be formed by: printing a first set of multipleparallel silicon nanowires on the substrate; printing a second set ofmultiple parallel silicon nanowires on the substrate over the first setof multiple parallel silicon nanowires such that the first set isperpendicular to the second set; forming a metal layer over the firstset and the second set; and performing a silicidation anneal.Alternatively, this metal silicide nanowire network can be formed byforming a silicide layer on the substrate; printing a first set ofmultiple parallel metal nanowires on the silicon layer; printing asecond set of multiple parallel metal nanowires on the silicon layerover the first set of multiple parallel metal nanowires such that thefirst set is perpendicular to the second set; and performing asilicidation anneal. Alternatively, this metal silicide nanowire networkcan be formed by: printing a first set of multiple parallel metalnanowires on the substrate; printing a second set of multiple parallelmetal nanowires on the substrate over the first set of multiple parallelmetal nanowires such that the first set is perpendicular to the secondset; forming a silicon layer over the first set and the second set; andperforming a silicidation anneal. In any case, after the silicidationanneal is performed, any unreacted metal or unreacted silicon can beselectively removed.

BRIEF DESCRIPTION OF THE DRAWINGS

The embodiments herein will be better understood from the followingdetailed description with reference to the drawings, which are notnecessarily drawn to scale and in which:

FIG. 1A is a cross-section diagram illustrating an embodiment of astructure with a metal silicide transparent conducting electrode;

FIG. 1B is a top view diagram of the structure of FIG. 1A;

FIG. 2A is a cross-section diagram illustrating another embodiment of astructure with a metal silicide transparent conducting electrode;

FIG. 2B is a top view diagram of the structure of FIG. 2A;

FIG. 3 is a flow diagram of a method of forming a structure as in FIGS.1A-1B or FIGS. 2A-2B;

FIG. 4A is a flow diagram illustrating a technique for performingprocess 306 of FIG. 3 to form the structure of FIGS. 1A-1B;

FIG. 4B is a flow diagram illustrating another technique for performingprocess 306 of FIG. 3 to form the structure of FIGS. 1A-1B;

FIG. 4C is a flow diagram illustrating yet another technique forperforming process 306 of FIG. 3 to form the structure of FIGS. 1A-1B;

FIG. 4D is a flow diagram illustrating yet another technique forperforming process 306 of FIG. 3 to form the structure of FIGS. 1A-1B;

FIG. 5A is cross-section diagram illustrating a partially completedstructure formed according to the technique of FIG. 4A;

FIG. 5B is cross-section diagram illustrating a partially completedstructure formed according to the technique of FIG. 4A;

FIG. 5C is cross-section diagram illustrating a partially completedstructure formed according to the technique of FIG. 4A;

FIG. 6A is cross-section diagram illustrating a partially completedstructure formed according to the technique of FIG. 4B;

FIG. 6B is cross-section diagram illustrating a partially completedstructure formed according to the technique of FIG. 4B;

FIG. 6C is cross-section diagram illustrating a partially completedstructure formed according to the technique of FIG. 4B;

FIG. 7A is cross-section diagram illustrating a partially completedstructure formed according to the technique of FIG. 4C;

FIG. 7B is cross-section diagram illustrating a partially completedstructure formed according to the technique of FIG. 4C;

FIG. 7C is cross-section diagram illustrating a partially completedstructure formed according to the technique of FIG. 4C;

FIG. 8A is cross-section diagram illustrating a partially completedstructure formed according to the technique of FIG. 4D;

FIG. 8B is cross-section diagram illustrating a partially completedstructure formed according to the technique of FIG. 4D;

FIG. 8C is cross-section diagram illustrating a partially completedstructure formed according to the technique of FIG. 4D;

FIG. 9A is a flow diagram illustrating a technique for performingprocess 306 of FIG. 3 to form the structure of FIGS. 2A-2B;

FIG. 9B is a flow diagram illustrating another technique for performingprocess 306 of FIG. 3 to form the structure of FIGS. 2A-2B;

FIG. 9C is a flow diagram illustrating yet another technique forperforming process 306 of FIG. 3 to form the structure of FIGS. 2A-2B;

FIG. 9D is a flow diagram illustrating yet another technique forperforming process 306 of FIG. 3 to form the structure of FIGS. 2A-2B;

FIG. 10A is cross-section diagram illustrating a partially completedstructure formed according to the technique of FIG. 9A;

FIG. 10B is cross-section diagram illustrating a partially completedstructure formed according to the technique of FIG. 9A;

FIG. 10C is top diagram illustrating the same partially completedstructure as shown in FIG. 10B;

FIG. 11A is cross-section diagram illustrating a partially completedstructure formed according to the technique of FIG. 9B;

FIG. 11B is top view diagram illustrating the same partially completedstructure as shown in FIG. 11A;

FIG. 11C is cross-section diagram illustrating a partially completedstructure formed according to the technique of FIG. 9B;

FIG. 12A is cross-section diagram illustrating a partially completedstructure formed according to the technique of FIG. 9C;

FIG. 12B is cross-section diagram illustrating a partially completedstructure formed according to the technique of FIG. 9C;

FIG. 12C is top view diagram illustrating the same partially completedstructure as shown in FIG. 12B;

FIG. 13A is cross-section diagram illustrating a partially completedstructure formed according to the technique of FIG. 9D;

FIG. 13B is a top view diagram illustrating the same partially completedstructure as shown in FIG. 13A; and

FIG. 13C is cross-section diagram illustrating a partially completedstructure formed according to the technique of FIG. 9D.

DETAILED DESCRIPTION

As mentioned above, transparent conductive electrodes (also referred toherein as transparent conducting films (TCFs)) are films that are bothoptically transparent allowing transmittance of, for example, 80% ormore of incident light and electrically conductive. Those skilled in theart will recognize that transparent conductive electrodes are oftenincorporated into modern devices, such as flat panel displays, touchpanels, solar cells, light emitting diodes (LEDs), organicoptoelectronics, etc. Currently, the material of choice for theseelectrodes is Indium Tin Oxide (ITO). Unfortunately, Indium isrelatively rare, making its use cost prohibitive in light of the globaldemand for transparent conductive electrodes. Additionally, transparentconductive electrodes formed from Indium may be overly brittle andtoxic. Therefore, there is a need in the art for a transparentconductive electrode structure and method of forming the structure thatis more commercially viable, robust, and safe to use.

In view of the foregoing, disclosed herein are embodiments of astructure with a metal silicide transparent conducting electrode, whichis commercially viable, robust, and safe to use (i.e., non-toxic) and,thus, optimal for incorporation into modern devices, such as flat paneldisplays, touch panels, solar cells, light emitting diodes (LEDs),organic optoelectronic devices, etc. Specifically, the structure cancomprise a substrate (e.g., a glass or plastic substrate) and atransparent conducting film (i.e., a transparent conducting electrode)on that substrate. The transparent conducting film can be made up of ametal silicide nanowire network. For example, in one embodiment, themetal silicide nanowire network can comprise multiple metal silicidenanowires fused together in a disorderly arrangement on the substrate.In another embodiment, the metal silicide nanowire network can comprisemultiple metal silicide nanowires fused together in a patterned grid onthe substrate. Also disclosed herein are various different methodembodiments for forming such a structure.

More particularly, referring to FIGS. 1A-1B and 2A-2B, disclosed hereinare embodiments 100, 200, respectively, of a structure with a metalsilicide transparent conducting electrode. Specifically, each of theembodiments 100, 200 of the structure can comprise a substrate 110, 210(e.g., a transparent glass or plastic substrate) and a transparentconducting film (i.e., a transparent conducting electrode) 120, 220 onthat substrate 110, 210.

The transparent conducting film 120, 220 can be made up of a metalsilicide nanowire network 150, 250 (i.e., a network of metal silicidenanowires 130, 230). For purposes of this disclosure, a nanowire is awire, fiber, high aspect ratio line, etc. with a diameter of the orderof a nanometer (nm) and, particularly, with a diameter that is tens ofnanometers or less. The conductivity of this transparent conducting film120, 220 can be relatively high, which means that the sheet resistivitycan be relatively low (e.g., 100-500 Ohms/square or less). Thetransparency of this transparent conducting film 120, 220 can also berelatively high (e.g., 60-99% using the AM 1.5 solar spectrum, as areference). Those skilled in the art will recognize that increasing thenanowire density (i.e., the average amount of nanowire material within aunit area) within the network 150, 250 of metal silicide nanowires 130,230 results in a corresponding decrease in transparency and increase inconductivity.

The metal silicide nanowire network 150, 250 can have various differentconfigurations, as shown in the different embodiments 100, 200.

For example, FIGS. 1A and 1B show cross-section and top view diagrams,respectively, of one embodiment 100 of the structure. In this embodiment100, the metal silicide nanowire network 150 can comprise multiple metalsilicide nanowires 130 in a disorderly arrangement (i.e., a jumble) onthe substrate 110 so that they essentially form a metal silicidenanowire mesh across the substrate 110. Additionally, as a result of thesilicidation process used to form these metal silicide nanowires 130(discussed in greater detail below with regard to the methodembodiments), they can be fused together at the interfaces 160 betweenintersecting nanowires (i.e., at the junctions between nanowires thatcross and contact each other), thereby reducing resistance across thenetwork 150. The diameter of these metal silicide nanowires 130 can berelatively small (e.g., can range, for example, between 2 nanometers(nm) and 200 nanometers (nm)), the aspect ratio (i.e., diameter tolength ratio) of the metal silicide nanowires 130 can be relatively high(e.g., can range between 1:100 and 1:1000 or greater) and the density ofthe nanowires can be optimized in order to allow for the greatest amountof transparency (e.g., 60-99% using the AM 1.5 solar spectrum, as areference) and still achieve a desired conductivity.

FIGS. 2A and 2B show cross-section and top view diagrams, respectively,of another embodiment 200 of the structure. In this embodiment 200, themetal silicide nanowire network 250 can comprise multiple metal silicidenanowires 230 patterned in a grid across the substrate 210. This gridcan, for example, comprise a first set of parallel metal silicidenanowires arrange in a first direction on the substrate 210 and a secondset of a parallel metal silicide nanowires stacked on the first set andarranged in a second direction that is essentially perpendicular to thefirst set. Additionally, as a result of the silicidation process used toform these metal silicide nanowires 230 (discussed in greater detailbelow with regard to the method embodiments), they can be fused togetherat the interfaces 260 between intersecting nanowires (i.e., at thejunctions between nanowires that cross and contact each other), therebyreducing resistance across the network 250. The diameter of these metalsilicide nanowires 230 can range, for example, between 2 nanometers (nm)and 60 nanometers (nm), the spacing between (i.e., the separationdistance between) adjacent parallel metal silicide nanowires 230 can beapproximately the same (i.e., uniform) and can range, for example,between 2 nanometers (nm) and 200 nanometers (nm) and the nanowiredensity can be optimized to allow for the greatest amount oftransparency (e.g., 60-99% using the AM 1.5 solar spectrum, as areference) and still achieve a desired conductivity (e.g., sheetresistivity of 100-500 Ohms/square or less).

In each of these embodiments 100, 200, the metal silicide nanowires 130,230 can comprise a single type of metal silicide nanowire. That is, allof the metal silicide nanowires 130, 230 on the substrate 110, 210 canbe made from the same silicided metal or metal alloy. Preferably, themetal can comprise a refractory metal or refractory metal alloy. Thus,for example, each of the metal silicide nanowires 130, 230 in the metalsilicide nanowire network 150, 250 can comprise a nickel (Ni) silicidenanowire, a cobalt (Co) silicide nanowire, a tungsten (W) silicidenanowire, a chromium (Cr) silicide nanowire, a platinum (Pt) silicidenanowire, a titanium (Ti) silicide nanowire, a molybdenum (Mo) silicidenanowire, a palladium (Pd) silicide nanowire or a refractory metal alloysilicide nanowire.

Alternatively, in each these embodiments 100, 200 the metal silicidenanowires 130, 230 in the metal silicide nanowire network 150, 250 cancomprise multiple different types of metal silicide nanowires. That is,some of the metal silicide nanowires 130, 230 on the substrate 110, 210can be made from a different silicided metal or metal alloy than others.Thus, for example, the metal silicide nanowires 130, 230 in the metalsilicide nanowire network 150, 250 can comprise a mix of nickel (Ni)silicide nanowires, cobalt (Co) silicide nanowires, tungsten (W)silicide nanowires, chromium (Cr) silicide nanowires, platinum (Pt)silicide nanowires, titanium (Ti) silicide nanowires, molybdenum (Mo)silicide nanowires, palladium (Pd) silicide nanowires and/or nanowirescomprising any refractory metal silicide. It should be noted that, asdiscussed in greater detail below with regard to the method embodiments,the incorporation of a mix of different types of nanowires into themetal silicide nanowire network is more easily achieved in theembodiment 100 as the process steps used to form this structure requiresuspension of nanowires in a solution prior to deposition onto thesubstrate.

In addition to the metal silicide nanowire network 150, 250, in each ofthe embodiments 100, 200, the transparent conducting film 120, 220 canfurther comprise a transparent polymer layer 140, 240. This transparentpolymer layer 140, 240 can coat the metal silicide nanowire network 150,250 and, more particularly, can fill in any gaps (i.e., spaces) betweenthe metal silicide nanowires 130, 230 within the metal silicide nanowirenetwork 150, 250 in order to provide protection and support. Optionally,the transparent polymer layer 140, 240 can be electrically conductive,thereby further increasing the conductivity of the transparentconducting film 120, 220. For example, the transparent polymer layer140, 240 can comprise a transparent non-conductive polymer layercontaining conductive particles (e.g., metal particles) or a transparentconductive polymer layer (e.g., a poly(3,4-ethylenedioxythiophene)PEDOT: poly(styrene sulfonate) PSS compound layer).

Referring to the flow diagram of FIG. 3, also disclosed herein aremethod embodiments for forming such a structure. The method embodimentscan comprise providing a substrate (e.g., a transparent glass or plasticsubstrate) (302) and forming a transparent conducting film (i.e., atransparent conducting electrode on that substrate (304).

Specifically, in order to form the transparent conducting film, a metalsilicide nanowire network can be formed on the substrate (306).

In one embodiment, as illustrated in FIGS. 1A-1B and discussed ingreater detail below, a metal silicide nanowire network 150 can beformed at process 306 such that it comprises multiple metal silicidenanowires 130 fused together in a disorderly arrangement (i.e., ajumble) on the substrate 110 so that they essentially form a metalsilicide nanowire mesh across the substrate 110. Such a metal silicidenanowire network 150 can be achieved using various different processingtechniques (see the flow diagrams of FIGS. 4A-4D).

For example, referring to the flow diagram of FIG. 4A, in order to formthe metal silicide nanowire network 150 of FIGS. 1A-1B a solution 502(i.e., a dispersion), which contains a plurality of silicon nanowires503 suspended in a solvent, can be applied to a substrate 501 (401, seeFIG. 5A). The solvent can subsequently be evaporated away such that onlythe silicon nanowires 503 remain on the surface of the substrate 501(see FIG. 5B). Next, a metal layer 504 can be formed (e.g., uniformly orselectively deposited by physical vapor deposition (PVD), deposited bychemical vapor deposition (CVD), etc.) over the silicon nanowires 503(402, see FIG. 5C). Finally, a silicidation anneal can be performed andany unreacted metal can be selectively removed (e.g., using a wet etchprocess) (403-404).

Alternatively, referring to the flow diagram of FIG. 4B, in order toform the metal silicide nanowire network 150 of FIGS. 1A-1B a metallayer 604 can be formed (e.g., deposited by electrodeposition, physicalvapor deposition (PVD) or chemical vapor deposition) on a substrate 601(411, see FIG. 6A). Then, a solution 602 (i.e., a dispersion), whichcontains a plurality of silicon nanowires 603 suspended in a solvent,can be applied to the metal layer 604 (412, see FIG. 6B). The solventcan subsequently be evaporated away such that only the silicon nanowires603 remain on the surface of the metal layer 604 (see FIG. 6C). Finally,a silicidation anneal can be performed and any unreacted metal can beselectively removed (e.g., by a wet etch process) (413-414).

With regard to the process steps set forth in the flow diagrams of FIGS.4A and 4B and illustrated in FIGS. 5A-5C and 6A-6C, respectively, itshould be noted that techniques for fabricating silicide nanowires(e.g., by growth on catalyst particles) are well-known in the art and,thus, are omitted from this specification in order to allow the readerto focus on the salient aspects of the invention. In any case, thesilicon nanowires 503, 603 can, for example, be formed so as to have adiameter ranging between 2 nanometers (nm) and 200 nanometers (nm) andan aspect ratio (i.e., diameter to length ratio) ranging between 1:100and 1:1000 or greater. The solution can comprise 0.01%-5% by weight ofsilicon nanowires 503, 603 in order to avoid agglomeration in thesolution and achieve a desired nanowire density following solventevaporation and, thereby to achieve a desired transparency (e.g., 60-99%using the AM 1.5 solar spectrum, as a reference) and a desiredconductivity (e.g., sheet resistivity of 100-500 Ohms/square or less) inthe resulting metal silicide nanowire network. It should further benoted that the solvent portion of the solution 502, 602 should be asolvent suitable for use in dispersion. Specifically, the solvent cancomprise any suitable fluid or mixture of fluids that is capable offorming a solution with the silicon nanowires 503, 603 and that can bevolatilized at a desired, relatively low temperature (e.g., a boilingpoint of less than 250° C.). An exemplary solution suitable forsuspending silicon nanowires 503, 603 can contain one part ethanol andtwo parts chloroform. Such a solution 502, 602 can be applied, forexample, by spray coating, roll coating, spin coating, etc.

Finally, the metal layer 504, 604 can comprise a refractory metal orrefractory metal alloy layer. Thus, for example, the metal layer 504,604 can comprise a layer of nickel (Ni), cobalt (Co), tungsten (W),chromium (Cr), platinum (Pt), titanium (Ti), molybdenum (Mo), palladium(Pd) or an alloy of any of these metals.

The above-described techniques can, for example, be used to form anickel silicide (NiSi) nanowire network with a resistivity 14-20 μohm-cmfrom a nickel layer and silicon nanowires. Specifically, a nickelsilicide (NiSi) nanowire network can be formed using a sinteringtemperature (i.e., a silicidation anneal temperature) of 400° C.-600° C.and is stable on silicon up to 650° C. In this case, 1.83 nm of siliconwill be consumed per nm of nickel. The resulting silicide thickness pernm of metal will be roughly 2.34 nm. Those skilled in the art willrecognize that these conditions will vary depending upon the metalmaterial used. However, it should be understood that the consumption ofthe silicon nanowires 503, 603 should be maximized during thesilicidation anneal such that a minimal etch is required to remove anyunreacted material. The technique should also attempt to maximizeconductivity.

Alternatively, referring to the flow diagram of FIG. 4C, in order toform the metal silicide nanowire network 150 of FIGS. 1A-1B a solution702 (i.e., a dispersion), which contains a plurality of metal nanowires703 suspended in a solvent, can be applied to a substrate 701 (421, seeFIG. 7A). The solvent can subsequently be evaporated away such that onlythe metal nanowires 703 remain on the surface of the substrate 701 (seeFIG. 7B). Next, a silicon layer 704 can be formed (e.g., deposited) overthe metal nanowires 703 (422, see FIG. 7C). Finally, a silicidationanneal can be performed and any unreacted silicon can be selectivelyremoved (e.g., by a dry etch process), thereby forming a metal silicidenanowire network 150 (423-424, see FIGS. 1A-1B and the detaileddescription above).

Alternatively, referring to the flow diagram of FIG. 4D, in order toform the metal silicide nanowire network 150 of FIGS. 1A-1B a siliconlayer 804 can be formed (e.g., deposited) on a substrate 801 (431, seeFIG. 8A). Then, a solution 802 (i.e., a dispersion), which contains aplurality of metal nanowires 803 suspended in a solvent, can be appliedto the silicon layer 804 (412, see FIG. 8B). The solvent cansubsequently be evaporated away such that only the metal nanowires 803remain on the surface of the silicon layer 804 (see FIG. 8C). Finally, asilicidation anneal can be performed and any unreacted silicon can beselectively removed (e.g., by a dry etch process) (433-434).

With regard to the process steps set forth in the flow diagrams of FIGS.4C and 4D and illustrated in FIGS. 7A-7C and 8A-8C, respectively, itshould be noted that techniques for fabricating metal nanowires (e.g.,by electrodeposition into template nanopores) are well-known in the artand, thus, are omitted from this specification in order to allow thereader to focus on the salient aspects of the invention. In any case,the metal nanowires 703, 803 can, for example, be formed so as to have adiameter ranging between 2 nanometers (nm) and 200 nanometers (nm) andan aspect ratio (i.e., diameter to length ratio) ranging between 1:100and 1:1000 or greater. Furthermore, the metal nanowires 703, 803 cancomprise a refractory metal or refractory metal alloy. Thus, forexample, the metal nanowires 703, 803 can comprise nanowires of nickel(Ni), cobalt (Co), tungsten (W), chromium (Cr), platinum (Pt), titanium(Ti), molybdenum (Mo), palladium (Pd) or an alloy of any of thesemetals. Furthermore, because the metal nanowires are suspended in asolution for deposition, a single type of metal nanowire can be used(i.e., all the nanowires can be made of the same type of metal or metalalloy) or different types of metal nanowires can be used (i.e., some ofthe metal nanowires can be made of one type of metal or metal alloy andother metal nanowires can be made of a different type of metal or metalalloy).

The solution can comprise 0.01%-5% by weight of metal nanowires 703, 803in order to avoid agglomeration in the solution and achieve a desirednanowire density following solvent evaporation and, thereby to achieve adesired transparency (e.g., 60-99% using the AM 1.5 solar spectrum, as areference) and a desired conductivity (e.g., sheet resistivity of100-500 Ohms/square or less) in the resulting metal silicide nanowirenetwork. It should further be noted that the solvent portion of thesolution 702, 802 should be a solvent suitable for use in dispersion.Specifically, the solvent can comprise any suitable fluid or mixture offluids that is capable of forming a solution with the metal nanowires703, 803 and that can be volatilized at a desired, relatively lowtemperature (e.g., a boiling point of less than 250° C.). An exemplarysolution suitable for suspending nickel nanowires can containisopropanol. Those skilled in the art will, however, recognize that theoptimal suspension solution will vary depending upon the type of metalnanowires used. In any case, this solution 702, 802 can be applied, forexample, by spray coating, roll coating, spin coating, etc.

Additionally, with regard to the process steps set forth in each of theflow diagrams of FIGS. 4A, 4B, 4C, and 4D, it should further be notedthat the silicidation anneal 403, 413, 423, 433 is a thermal annealperformed in order to cause metal atoms from the metal layer or metalnanowires, as applicable, to react with the silicon material from thesilicon layer or silicon nanowires, as applicable. Those skilled in theart will recognize that the specifications for the silicidation anneal(e.g., the anneal temperature and duration) will vary depending uponmaterials used, their thicknesses, etc. Thus, the thickness of thesilicon layer 704, 804 should be chosen in relation to the thickness ofthe metal nanowires 703, 803 with the objective being to maximize theconductivity of the resulting metal silicide nanowire network andmaximize the consumption/reaction of silicon during the silicidationprocess. In this case, the thickness of the silicon consumed can rangefrom 0.5 nm to 4 nm per nm of metal depending on the metal chosen.Furthermore, sintering temperatures (i.e., silicidation annealtemperatures) as low as 250° C. and as high as 1000° C. can be used. Asa result, a metal silicide nanowire network 150 is created with multiplemetal silicide nanowires 130 fused together at the interfaces 160between intersecting nanowires (i.e., at the junctions between nanowiresthat cross and contact each other), thereby reducing resistance acrossthe network 150 (see FIGS. 1A-1B and the detailed description above).

Finally, it should be noted that in one embodiment the substrate 501,601, 701, 801, as shown in FIGS. 5A-5C, 6A-6C, 7A-7C, and 8A-8C,respectively, on which the metal silicide nanowire network is initiallyformed can be the same substrate (e.g., a transparent glass or plasticsubstrate) as shown in the final structure of FIGS. 1A-1B. However,alternatively, this substrate 501, 601, 701, and 801 can comprise adummy substrate (i.e., a sacrificial substrate). The dummy substrate cancomprise a dielectric substrate (e.g., a silicon dioxide (SiO2)substrate) or other suitable substrate onto which the metal silicidenanowire network can be formed and subsequently removed. In this case,at some point during processing (e.g., after the silicidation anneal,but prior to the selective removal of any unreacted metal or silicon)the metal silicide nanowire network can be transferred from the dummysubstrate to the final substrate (i.e., to the transparent glass orplastic substrate). Those skilled in the art will recognize that thistransfer can be performed, for example, using thermal tape to lift themetal silicide nanowire network off the dummy substrate and place it onthe final substrate.

In another embodiment, as illustrated in FIGS. 2A-2B and discussed ingreater detail below, a metal silicide nanowire network 250 can beformed at process 306 such that it comprises multiple metal silicidenanowires 230 fused together in a patterned grid on the substrate 210.Such a metal silicide nanowire network 250 can be achieved using variousdifferent processing techniques (see the flow diagrams of FIGS. 9A-9D).

For example, referring to the flow diagram of FIG. 9A, in order to formthe metal silicide nanowire network 250 of FIGS. 2A-2B a metal layer1004 can be formed (e.g., uniformly or selectively deposited byelectrodeposition, physical vapor deposition (PVD) or chemical vapordeposition) on the substrate 1001 (901, see FIG. 10A). Then, a first setof multiple parallel silicon nanowires 1003.1 can be printed on themetal layer 1004 and a second set of multiple parallel silicon nanowires1003.2 can be printed on the metal layer 1004 over the first set ofmultiple parallel silicon nanowires 1003.1 such that the first set isperpendicular to the second set (902, see FIGS. 10B and 10C). Finally, asilicidation anneal can be performed and any unreacted metal can beselectively removed (e.g., using a wet etch process)(903-904).

Alternatively, referring to the flow diagram of FIG. 9B, in order toform the metal silicide nanowire network 250 of FIGS. 2A-2B, a first setof multiple parallel silicon nanowires 1103.1 can be printed on asubstrate 1101 and a second set of multiple parallel silicon nanowires1103.2 can be printed on the substrate 1101 over the first set ofmultiple parallel silicon nanowires 1103.1 such that the first set isperpendicular to the second set (911, see FIGS. 11A and 11B). Then, ametal layer 1104 can be formed (e.g., uniformly or selectively depositedby physical vapor deposition (PVD), chemical vapor deposition (CVD),etc.) over the first set and the second set (902, see FIG. 11C).Finally, a silicidation anneal can be performed and any unreacted metalcan be selectively removed (e.g., using a wet etch process) (913-914).

With regard to the process steps set forth in the flow diagrams of FIGS.9A and 9B and illustrated in FIGS. 10A-10C and 11A-11C, respectively, itshould be noted that the metal layer 1004, 1104 can comprise arefractory metal or refractory metal alloy layer. Thus, for example, themetal layer 1004, 1104 can comprise a layer of nickel (Ni), cobalt (Co),tungsten (W), chromium (Cr), platinum (Pt), titanium (Ti), molybdenum(Mo), palladium (Pd) or an alloy of any of these metals.

Alternatively, referring to the flow diagram of FIG. 9C, in order toform the metal silicide nanowire network 250 of FIGS. 2A-2B a siliconlayer 1204 can be formed (e.g., deposited) on a substrate 1201 (921, seeFIG. 12A). Then, a first set of multiple parallel metal nanowires 1203.1can be printed on the silicon layer 1204 and a second set of multipleparallel metal nanowires 1203.2 can be printed on the silicon layer 1204over the first set of multiple parallel metal nanowires 1203.1 such thatthe first set is perpendicular to the second set (922, see FIGS. 12B and12C). Finally, a silicidation anneal can be performed and any unreactedsilicon can be selectively removed (e.g., using a dry etchprocess)(923-924).

Alternatively, referring to the flow diagram of FIG. 9D, in order toform the metal silicide nanowire network 250 of FIGS. 2A-2B, a first setof multiple parallel metal nanowires 1303.1 can be printed on asubstrate 1301 and a second set of multiple parallel metal nanowires1303.2 can be printed on the substrate 1301 over the first set ofmultiple parallel metal nanowires 1303.1 such that the first set isperpendicular to the second set (931, see FIGS. 13A and 13B). Then, asilicon layer 1304 can be formed (e.g., deposited) over the first setand the second set (932, see FIG. 13C). Finally, a silicidation annealcan be performed and any unreacted silicon can be selectively removed(e.g., using a dry etch process) (913-914).

With regard to the process steps set forth in the flow diagrams of FIGS.9C and 9D and illustrated in FIGS. 12A-12C and 13A-13C, respectively, itshould be noted that the metal nanowires 1203.1-1203.2, 1303.1-1303.2can comprise a refractory metal or refractory metal alloy. Thus, forexample, the metal nanowires 1203.1-1203.2, 1303.1-1303.2 can comprisenanowires of nickel (Ni), cobalt (Co), tungsten (W), chromium (Cr),platinum (Pt), titanium (Ti), molybdenum (Mo), palladium (Pd) or analloy of any of these metals. Furthermore, because the metal nanowiresare layered during multiple printing processes, a single type of metalnanowire can be used (i.e., all the nanowires can be made of the sametype of metal or metal alloy) or different types of metal nanowires canbe used (i.e., the first set of metal nanowires can be made of one typeof metal or metal alloy and the second set can be made of a differenttype of metal or metal alloy).

With regard to the process steps set forth in each of the flow diagramsof FIGS. 9A, 9B, 9C, and 9D, it should be understood that techniques forprinting sets of parallel nanowires, including silicon nanowires andmetal nanowires, onto a substrate are well known in the art (e.g., byusing a superlattice template to form the nanowires, as shown in U.S.Patent Application Publication 2005/02560276 of Heath et al., publishedon Nov. 10, 2005 and incorporated herein by reference) and, thus, thedetails of these techniques are omitted from this specification in orderto allow the reader to focus on the salient aspects of the invention.However, these techniques should be performed such that the diameter ofthe metal nanowires range, for example, between 2 nanometers (nm) and200 nanometers (nm) and that the spacing between (i.e., the separationdistance between) adjacent parallel metal nanowires is maximized (e.g.,between 2 nanometers (nm) and 200 nanometers (nm)) so that the resultingnanowire density is optimized to allow for the greatest amount oftransparency (e.g., 60-99% using the AM 1.5 solar spectrum, as areference) and still achieve a desired conductivity (e.g., sheetresistivity of 100-500 Ohms/square or less).

It should also be noted that, as discussed with regard to the previousmethod embodiment, the silicidation anneal is a thermal anneal performedin order to cause metal atoms from the metal layer or metal nanowires,as applicable, to react with the silicon material from the silicon layeror silicon nanowires, as applicable. Those skilled in the art willrecognize that the specifications for the silicidation anneal (e.g., theanneal temperature and duration) will vary depending upon materialsused, their thicknesses, etc. As a result, a metal silicide nanowirenetwork 250 is created with multiple metal silicide nanowires 230 fusedtogether at the interfaces 260 between intersecting nanowires (i.e., atthe junctions between nanowires that cross and contact each other),thereby reducing resistance across the network 250 (see FIGS. 2A-2B andthe detailed description above).

Finally, it should be noted that in one embodiment the substrate 1001,1101, 1201, 1301, as shown in FIGS. 10A-10C, 11A-11C, 12A-12C, and13A-13C, respectively, on which the metal silicide nanowire network isinitially formed can be the same substrate (e.g., a transparent glass orplastic substrate) as shown in the final structure of FIGS. 2A-2B.However, alternatively, this substrate 1001, 1101, 1201, and 1301 cancomprise a dummy substrate (i.e., a sacrificial substrate). The dummysubstrate can comprise a dielectric substrate (e.g., a silicon dioxide(SiO2) substrate) or other suitable substrate onto which the metalsilicide nanowire network can be formed and subsequently removed. Inthis case, at some point during processing (e.g., after the silicidationanneal, but prior to the selective removal of any unreacted metal orsilicon) the metal silicide nanowire network can be transferred from thedummy substrate to the final substrate (i.e., to the transparent glassor plastic substrate). Those skilled in the art will recognize that thistransfer can be performed, for example, using thermal tape to lift themetal silicide nanowire network off the dummy substrate and place it onthe final substrate.

Referring again to the flow diagram of FIG. 3 and to the embodiments100, 200 of the structure as shown in FIGS. 1A-1B and 2A-2B, optionally,after the metal silicide nanowire network 150, 250 is formed on thesubstrate 110, 210, a transparent polymer layer 140, 240 can be formedin order to provide protection and support to the network 150, 250(308). Specifically, this transparent polymer layer 140, 240 can beformed (e.g., deposited) so as to coat the metal silicide nanowirenetwork 150, 250 and, more particularly, so as to fill in any gaps(i.e., spaces) between the metal silicide nanowires 130, 230 of themetal silicide nanowire network 150, 250. Optionally, this transparentpolymer layer 140, 240 can be electrically conductive, thereby furtherincreasing the conductivity of the transparent conducting film 120, 220.For example, the transparent polymer layer 140, 240 can comprise atransparent non-conductive polymer layer containing conductive particles(e.g., metal particles) or a transparent conductive polymer layer (e.g.,a poly(3,4-ethylenedioxythiophene) PEDOT: poly(styrene sulfonate) PSScompound layer).

The resulting transparent conducting film 120, 220 (i.e., the metalsilicide transparent conducting film) is safer and more commerciallyviable than prior art transparent conducting films because itincorporates materials that are non-toxic and commonly used insemiconductor manufacturing. The resulting transparent conducting film120, 220 is also more robust than prior art transparent conducting filmsbecause the nanowires that form the network are less brittle and do notsimply contact each other but rather are fused together as a result ofthe silicidation process and, thus, are subject to less resistance atthe nanowire to nanowire interfaces. Consequently, the resultingstructure of embodiments 100, 200 is optimal for incorporation intomodern devices, such as flat panel displays, touch panels, solar cells,light emitting diodes (LEDs), organic optoelectronic devices, etc.

It should be understood that the terminology used herein is for thepurpose of describing particular embodiments only and is not intended tobe limiting of the embodiments. For example, the singular forms “a”,“an” and “the” are intended to include the plural forms as well, unlessthe context clearly indicates otherwise. Additionally, the terms“comprises”, “comprising,” “includes,” and/or “including,” when used inthis specification, specify the presence of stated features, integers,steps, operations, elements, and/or components, but do not preclude thepresence or addition of one or more other features, integers, steps,operations, elements, components, and/or groups thereof. It shouldfurther be understood that terms such as “right”, “left”, “vertical”,“horizontal”, “top”, “bottom”, “upper”, “lower”, “under”, “below”,“underlying”, “over”, “overlying”, “parallel”, “perpendicular”, etc.,used herein are understood to be relative locations as they are orientedand illustrated in the drawings (unless otherwise indicated). Terms suchas “touching”, “on”, “in direct contact”, “abutting”, “directly adjacentto”, “immediately adjacent to”, etc., mean that at least one elementphysically contacts another element (without other elements separatingthe described elements). Finally, it should be understood that thecorresponding structures, materials, acts, and equivalents of all meansor step plus function elements in the claims below are intended toinclude any structure, material, or act for performing the function incombination with other claimed elements as specifically claimed. Theabove-description of the embodiments has been presented for purposes ofillustration and description and it is not intended to be exhaustive.Many modifications and variations to the disclosed embodiments will beapparent to those of ordinary skill in the art without departing fromthe scope and spirit of those embodiments.

Therefore, disclosed above are embodiments of a structure with a metalsilicide transparent conducting electrode, which is commercially viable,robust, and safe to use (i.e., non-toxic) and, thus, optimal forincorporation into modern devices, such as flat panel displays, touchpanels, solar cells, light emitting diodes (LEDs), organicoptoelectronic devices, etc. Specifically, the structure can comprise asubstrate (e.g., a glass or plastic substrate) and a transparentconducting film (i.e., a transparent conducting electrode) on thatsubstrate. The transparent conducting film can be made up of a metalsilicide nanowire network. For example, in one embodiment, the metalsilicide nanowire network can comprise multiple metal silicide nanowiresfused together in a disorderly arrangement on the substrate. In anotherembodiment, the metal silicide nanowire network can comprise multiplemetal silicide nanowires fused together in a patterned grid on thesubstrate. Also disclosed herein are various different methodembodiments for forming such a structure.

What is claimed is:
 1. A method, comprising: forming, on a substrate, ametal silicide nanowire network for a transparent conducting film byforming a metal layer on said substrate; applying a solution containingsilicon nanowires onto said metal layer; evaporating said solution suchthat only the silicon nanowires remain on a surface of said metal layer;performing a silicidation anneal on said silicon nanowires and saidmetal layer to cause metal atoms from the metal layer to react withsilicon material in said silicon nanowires, thus forming said metalsilicide nanowire network; and filling gaps within said metal silicidenanowire network by coating said metal silicide nanowire network with anelectrically conductive transparent polymer layer thereby increasingelectrical conductivity of the transparent conducting film, wherein saidconductive transparent polymer layer is made of a transparentnon-conductive polymer layer containing conductive particles, whereinsaid electrically conductive transparent polymer layer provides supportto said metal silicide nanowire network.
 2. The method according toclaim 1, further comprising: after said performing said silicidationanneal, transferring said metal silicide nanowire network from saidsubstrate to another substrate and selectively removing remainingunreacted metal and unreacted silicon.
 3. The method according to claim1, said electrically conductive transparent polymer layer comprising aconductive polymer layer.
 4. The method according to claim 1, said metalsilicide nanowire network comprising a plurality of metal silicidenanowires fused together in a disorderly arrangement on said substrate.5. The method according to claim 4, said metal silicide nanowirescomprising a plurality of nanowires of different types of metal.
 6. Themethod according to claim 1, said metal silicide nanowires comprisingany of nickel (Ni) silicide nanowires, cobalt (Co) silicide nanowires,tungsten (W) silicide nanowires, chromium (Cr) silicide nanowires,platinum (Pt) silicide nanowires, titanium (Ti) silicide nanowires,molybdenum (Mo) silicide nanowires, and palladium (Pd) silicidenanowires.
 7. A method, comprising: forming, on a substrate, a metalsilicide nanowire network for a transparent conducting film by forming ametal layer on said substrate; applying a solution containing siliconnanowires onto said metal layer; evaporating said solution such thatonly the silicon nanowires remain on a surface of said metal layer;performing a silicidation anneal on said silicon nanowires and saidmetal layer to cause metal atoms from the metal layer to react withsilicon material in said silicon nanowires, thus forming said metalsilicide nanowire network; after said performing said silicidationanneal, selectively removing remaining unreacted metal and unreactedsilicon; and filling gaps within said metal silicide nanowire network bycoating said metal silicide nanowire network with an electricallyconductive transparent polymer layer thereby increasing electricalconductivity of the transparent conducting film, wherein said conductivetransparent polymer layer is made of a transparent non-conductivepolymer layer containing conductive particles, wherein said electricallyconductive transparent polymer layer provides support to said metalsilicide nanowire network.
 8. The method according to claim 7, furthercomprising: after said performing said silicidation anneal, transferringsaid metal silicide nanowire network from said substrate to anothersubstrate and selectively removing remaining unreacted metal andunreacted silicon.
 9. The method according to claim 7, said electricallyconductive transparent polymer layer comprising a conductive polymerlayer.
 10. The method according to claim 7, said metal silicide nanowirenetwork comprising a plurality of metal silicide nanowires fusedtogether in a disorderly arrangement on said substrate.
 11. The methodaccording to claim 10, said metal silicide nanowires comprising aplurality of nanowires of different types of metal.
 12. The methodaccording to claim 7, said metal silicide nanowires comprising any ofnickel (Ni) silicide nanowires, cobalt (Co) silicide nanowires, tungsten(W) silicide nanowires, chromium (Cr) silicide nanowires, platinum (Pt)silicide nanowires, titanium (Ti) silicide nanowires, molybdenum (Mo)silicide nanowires, and palladium (Pd) silicide nanowires.